3. DISCUSSION

The discovery of dark energy has presented both observational
and theoretical cosmologists with a win-win scenario. On the
observational side, we will either verify to high precision the
existence of a truly constant vacuum energy representing a new
fundamental constant of nature and a potentially crucial clue
to the reconciliation of gravity with quantum field theory,
or we will detect variations
in the dark energy density which indicate either a new dynamical
component or an alteration of general relativity itself. On
the theoretical side, we have been given invaluable insight into
one of the most perplexing issues in theoretical physics (the
cosmological constant problem), and we are now faced with a brand
new issue (the coincidence problem) whose resolution will
necessarily involve exciting new theoretical developments.

Nobody who took arguments of naturalness and fine-tuning seriously
would have expected to discover a small but nonzero dark energy
density(1)
We should not conclude from this that such arguments
have no value, but that we should always be prepared for
surprises. One way of characterizing our current inventory of
the universe is to divide it into ordinary baryonic matter,
comprising 5% of the energy density of the universe, and
a "dark sector" comprising the remaining 95%. In this
classification, the role of the recent discoveries has been to
reveal that the dark sector includes at least two distinct
components, the dark matter and the dark energy. Who is to say
that future experiments will not reveal further structure within
this sector, perhaps including interesting interactions between
components? It is safe to say that the future of dark physics
looks very bright.

Acknowledgments

This work was supported in part by the
U.S. Dept. of Energy, the Alfred P. Sloan Foundation, and the David
and Lucile Packard Foundation.

1 This is a rhetorical exaggeration.
It has been pointed out to me, quite correctly,
that people who took naturalness arguments seriously might have
expected a nonzero cosmological constant if they also took the
anthropic principle seriously.
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